1. Field
The present invention relates generally to data communication, and more specifically to techniques for retransmitting packets by a Radio Link Protocol (RLP) in CDMA communication systems.
2. Background
Wireless communication systems are widely deployed to provide various types of services such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users, and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of systems, including increased system capacity.
To improve the reliability of data transmission, many CDMA systems employ a retransmission mechanism provided by a Radio Link Protocol (RLP) that resides above the physical layer. An RLP entity at the receiver (i.e., the receiver RLP) is provided with RLP frames by a lower-level entity, with each RLP frame being uniquely identified by its assigned sequence number. Since the RLP frames to be transmitted are assigned sequential sequence numbers, the receiver RLP is able to determine whether or not any RLP frames are missing by looking at the sequence numbers of the received RLP frames. In the absence of any other retransmission mechanism, the RLP frames may be transmitted serially to the receiver, and any RLP frame detected to be missing by the receiver RLP may be immediately reported to the transmitter via a transmission of a negative acknowledgment (NAK). The missing RLP frames may thereafter be retransmitted to the receiver.
To provide improved packet data transmission capability, some newer-generation CDMA systems (e.g., cdma2000 Release C) employ another retransmission mechanism provided by a hybrid automatic retransmission control function (HARQ-CF) that resides between the RLP and the physical layer. The HARQ-CF defined by cdma2000 is able to transmit multiple HARQ packets in parallel, with each HARQ packet including one or more RLP frames. Moreover, because each HARQ packet may be transmitted/retransmitted once or multiple times by the HARQ-CF, the HARQ packets may be recovered in an unknown order by the HARQ-CF entity at the receiver (i.e., the receiver HARQ-CF). Thus, RLP frames may be provided out of sequence to the receiver RLP. In addition, because of retransmission by the HARQ-CF, the receiver RLP may have to wait a long time period before it can determine with confidence that a given missing RLP is really lost. The HARQ-CF retransmission may thus severely impact the performance of the RLP retransmission.
There is therefore a need in the art for an RLP retransmission scheme that can operate efficiently in conjunction with the underlying HARQ-CF retransmission mechanism.